1. Field of the Invention
This invention relates to an amplifying circuit, and in particular it relates to an amplifying circuit for obtaining two input signals for a synchronous video detector in a television receiver.
2. Description of the Prior Art
In the past a diode detector or a transistor detector has usually been used as a video detector in a television receiver. However, such detectors have the disadvantage that their input-output characteristic is nonlinear due to the nonlinearity of the diode or the transistor. Because of this nonlinearity, if a video intermediate frequency (IF) signal and a sound IF signal are supplied to the detector, the resulting output signal not only includes a video signal and sound IF signal having a carrier frequency of 4.5 MHz, but in addition a beat signal at a frequency of 920 KHz, which is the frequency difference of the 4.5 MHz sound IF signal and the color signal of 3.58 MHz. Many other harmonic signals are also produced, but the frequencies of these harmonic signals are outside of the video frequency band, so they have no adverse influence on the reproduced image. The 920 KHz beat signal is within the video frequency band and is therefore reproduced in the image as a noise signal. In order to eliminate the beat noise signal, it has previously been proposed either to connect in the video signal path a trap circuit tuned to the frequency of the beat signal, or to avoid supplying a sound IF signal at the original intermediate frequency to the video detector. The first of these two proposals decreases the quality of the frequency characteristic of the video signal, and the second proposal makes it necessary to provide another detecting circuit in order to obtain a sound IF signal having a carrier frequency of 4.5 MHz.
It has also been proposed to use a synchronous video detector in order to eliminate the foregoing advantages. By using a synchronous video detector, it is possible to eliminate the above defects, but the resultant circuit requires many components, and it is very difficult to fabricate it in an integrated circuit.